Rotatable magnetic systems for transforming audio currents



y 1962 A. M SPRINGER ROTATABLE MAGNETIC SYSTEMS FOR TRANSFORMING AUDIOCURRENTS Original Filed Oct. 29. 1956 3 Sheets-Sheet 1 [NI/EDITOR W ly1962 A. M. SPRINGER ROTATABLE MAGNETIC SYSTEMS FOR TRANSFORMING AUDIOCURRENTS Original Filed Oct. 29, 1956 3 Sheets-Sheet 2 July 31, 1962 AM. SPRINGER $047,673

ROTATABLE MAGNETIC SYSTEMS FOR TRANSFORMING AUDIO CURRENTS OriginalFiled Oct. 29, 1956 3 SheetsSheet 5 IYIIIII.

rl-b'lllld I /N VE/VTOR M G/Wm United States Patent Ofiice Patented July31, 1962 3 047 673 ROTATABLE MAGNE EIC SYSTEMS FOR TRANS- FORMENG AUDIOCUNTS Anton M. Springer, Hamburg vor der Hohe, Germany,

assignor to Telefonhau und Normalzeit G.m.b.H.,.

This is a division of application Ser. No. 618,766, filed October 29,1956, for Rotatable Magnetic Systems for Transforming Audio Currents,now Patent No. 3,022,- 383, issued February 20, 1962.

It is one object of this invention to provide improved playback headsmaking it possible to change, i.e. to accelerate or decelerate, thespeed of reproduction or playback of audio records without changing thepitch of the sounds involved.

It is another object of the invention to provide a device capable ofreproducing records of speech, or music, at a more rapid, or less rapid,speed than that at which the original record thereof was made, Withoutaffecting the pitch, and other tonal qualities, of the sounds involved.Devices of this character may be used by stenographers for reducing thespeed of play back of dictation relative to the speed of the originaldictation, or to facilitate the understanding of speech in foreignlanguages by reducing the speed of playback. Acceleration ofreproduction may be desired in cases where the skill of musicians is notsuflicient to keep pace with exacting speed requirements. Devices forchanging the speed of playback of sound are also needed forsynchronizingsound films, as necessary where sound has been recorded separately fromthe motion picture. Another very important application of transformingaudio currents with a view to change the speed of reproduction withoutchanging the tonal qualities of the record is the broadcasting ofintelligence and of music where it is often desired to compress, or tostretch, the time of playback of a given record.

In order to reproduce the tonal qualities of a record with a high degreeof fidelity, the relative velocity between the playback head, which maybe a magnetic head, where magnetic tape with a sound track thereon isbeing used for recording sound, and the record must be the same as therelative velocity between the recording head and the record at the timethe original record is being made. A change of a few percent of therelative speed at reproduction in regard to the relative speed atoriginal recording changes the tonal qualities of the reproduction sodrastically as to make it impossible to even recognize a well knownvoice. A change in the speed of reproduction or playback can be achievedby maintaining, during reproduction of a sound record, the relativevelocity between reproducing head and record exactly the same as therelative velocity between recording head and record during the originalrecording operation, but increasing or decreasing, as the case may be,the absolute velocity at which the record is being moved. An increase ofthe absolute velocity of the record results in shortening, and adecrease of the absolute velocity of the record results in lengthening,of the time required for reproducing a given sound record. Varying theabsolute 'velocity of a record at will while maintaining a predeterminedrelative velocity between reproduction head and record calls forelectro-acoustic systems having rotatable multiple playback heads, e.g.multiple magnetic playback heads.

It is one object of this invention to provide improved electro-acousticplayback systems of the aforementioned character, and more particularlyto provide such systems having improved rotatable multiple magneticplayback heads.

Where the absolute velocity of a record is being increased to shortenthe time of playback, and the relative velocity between reproductionhead and record is maintained at a given value to maintain the pitch ofthe sounds involved, certain increments of the sound track are beingperiodically omitted, or deleted. Similarly, where the absolute velocityof a record is being decreased to lengthen the time of playback,periodic increments of the sound track are being repeated, i.e. playedtwice, during reproduction of the record. The shortest audible sound isa sound whose duration is 40 milliseconds. The duration of theincrements on the sound track which are being deleted, or repeated, asthe case may be, must be less than 40 milliseconds, to preclude thesedeletions or repetitions from being noticed. Considering a magnetic tapemoving at a velocity of 76 cms. per sec., the distance between twoadjacent magnetic heads of a multiple magnetic reproduction head, i.e.the distance between the pole gaps thereof, must be 31 millimeters tocomply with the requirement that the increments of the sound trackdeleted, or repeated, during playback not exceed 40 milliseconds. At atape velocity of 38 cms. per second the distance between adjacent polegaps of a rotatable multiple magnetic reproduction head must be as smallas 15.5 millimeters, and at a tape velocity of 19 cms. per second thedistance between adjacent pole gaps of a rotatable multiple magneticplayback head must be as small as 7.75 millimeters. It is very difficultand expensive to manufacture rotatable multiple reproduction headshaving an excessively small spacing between the individual constituentplayback heads thereof.

It is, therefore, another general object of this invention to provideimproved electro-acoustic reproduction systems of the aforementionedcharacter having rotatable multiple playback heads which lend themselvesto be readily manufactured at relatively small cost, however narrow thespacing between adjacent individual playback heads or pole gaps may be.

In the preferred embodiments of this invention the sound record is inthe form of magnetic tape with a sound track thereon, and the play backhead a magnetic head.

It is, therefore, a special object of this invention to provide improvedtape playback systems having rotatable multiple magnetic playback headswhich lend themselves to be readily manufactured at relatively smallcost, however narrow the spacing between adjacent poles or air gapsthereof may be.

An important problem in connection with playback speed control systemsof the aforementioned character is the nature of the drive thereof. Theconstant relative speed of the sound record relative to the playbackhead can readily be achieved by driving the latter by means of a motorhaving a constant number of revolutions, e.g. a synchronous motor. Sucha motor is one of several available types of motors having a constantrelative velocity between rotor and stator. To achieve the speed controlrequired for varying the absolute velocity of thesound record, thestator of the synchronous motor may be driven by an auxiliary motor andan intermediary gear having a variable gear ratio, the rotatable statorbeing used to drive the sound record. As an alternative, the rotatable.stator of a synchronous motor may be coupled with, and driving, amultiple playback head, and the rotor coupled with, and driving, acapstan for advancing a magnetic tape, in which case the rotor will bedriven by an auxiliary motor and intermediate gears having a variablegear ratio.

It is a further object of this invention to provide improved playbackspeed control systems comprising a synchronous motor, or other motorhaving a fixed relative velocity between rotor and stator, whose statordrives the multiple playback head and whose rotor drives a sound recordand is being driven by an auxiliary motor by the aoazeve intermediary ofa variable speed drive, i.e. a transmission having a variable gearratio.

It is a further special object of this invention to provide improvedmagnetic tape playback systems comprising an electric motor having astator driving a multiple magnetic playback head and a rotor driving acapstan engaging magnetic tape and advancing the same at predeterminedspeeds wherein the rotor of the synchronous motor is being driven by anauxiliary motor by the intermediary of a variable speed drive.

It will be understood that the terms. rotor and stator which have beenused in the foregoing because their use is conventional in the art fordesignating the two parts of an electric motor, such as a synchronousmotor, have lost their original significance in the present contextinasmuch as the stator is not a static but a rotatable part used todrive the multiple magnetic playback head, or other muL tiple rotatablesound reproduction head.

A further object of the invention is to provide improved magnetic tapeplayback systems having an electric drive of the aforementionedcharacter or, in other words, an electrical differential of theaforementioned character, which drive has a high degree of stability,and whose magnetic leakage flux does not tend to produce noise in therotatable multiple magnetic playback head.

Other object and advantages of the invention will become apparent asthis specification proceeds, and the features of novelty whichcharacterize the invention will be pointed out with particularity in theappended claims forming part of this specification.

For a better understanding of the invention reference may be had to theaccompanying drawings in which:

FIG. 1 is an isometric view of an electro-acoustic playback systemembodying the invention;

FIG. 1a is a top plan view of a rotatable multiple magnetic playbackhead such as used in the structure of FIG. 1;

FIG. 2 is an isometric view of the structure shown in FIG. 1a;

FIG. 3 is a top plan view of a rotatable magnetic playback headcomprising six pole pieces forming three magnetic gaps;

FIG. 4 is a section along 4--4 of FIG. 3;

FIG. 5 is a side elevation of the magnetic laminae forming the poles ofthe structure shown in FIGS. 3 and 4;

FIG. 6 is an isometric view of the magnetic core structure of a dualmagnetic playback head embodying the invention, the magnetizing windingbeing omitted in this figure; 7

FIG. 7 is an isometric view of the magnetic core structure of a triplemagnetic playback head embodying the invention, the magnetizing windingbeing likewise omitted in this figure;

FIG. 8 is a top plan view of a rotatable magnetic playback headembodying this invention comprising four pairs of spaced pole piecesforming four magnetic gaps;

FIG. 9 is a side elevation of drive means for a rotatable multiplemagnetic playback head and for magnetic tape with a sound track thereon;

FIG. 10 is a side elevation of substantially the same drive means for arotatable magnetic playback head and for magnetic tape as shown in FIG.1; and

FIG. 11 is a side elevation of a modification of the structures shown inFIGS. 9 and 10.

Referring now to the drawings, and more particularly to FIGS. 1 and 10thereof, reference character 1 has been applied to generally indicate arotatable multiple magnetic playback head. The structural details ofplayback head 1 are shown in FIGS 1:: to 8, inclusive, and will bedescribed in connection with these figures. Magnetic playback head 1 isdriven by shaft 2. Gear 3 is fixedly mounted on shaft 2 and driven bygear 4. Magnetic tape 5 is wound along a predetermined angle around thecylindrical surface of magnetic playback head 1, and

moves in the direction of arrows 6 and 7 from a supply reel (not shown)on the left to a take up reel (not shown) on the right of FIGURE 1. Thetape drive comprises capstan 8 fixedly mounted on driving shaft 9parallel to driven shaft 2, and the pressure roller 19. The tape drivefurther includes a pair of synchronous motors generally indicated byreference numerals 13 and 14. The axes of rotation and the shafts ofmotors 13 and 14 are arranged parallel to each other, and parallel tothe driving shaft 2 of magnetic playback head 1. Motors 13 and 14 arearranged between a pair of parallel plates 12 and 15 forming part of amounting frame structure or chassis. Synchronous motor 14 comprises therotor 14c mounted on, or coupled with, shaft 9 supported in bearings 12band 15b provided in plates 12 and 15, and stator 14a. Friction plate 17is arranged coaxially with respect to rotor 14c and stator 14a andfixedly mounted on the former for joint rotation therewith. Synchronousmotor 13--which is an auxiliary motorcomprises the rotor 13c mounted onshaft 13b supported in bearings 12c and 15a in frame plates 12 and 1d.Motor 13 further comprises the stator 13a fixedly mounted on thechassis, whereas stator 140 of motor 14 is rotatable about shaft 9, andthus adapted to rotate relative to chassis plates 14, 15. Friction plate16 is arranged coaxially with respect to rotor 13a and stator 13c, andfixedly mounted on the former the joint rotation therewith. Shaft 18supporting friction rollers 19, 20 is supported by a bearing rod 21adapted to be shifted selectively either to the left, or to the right,as indicated by the arrows 22 and 23. Shaft 18 is arranged at rightangles to shafts 9 and 13b, and rollers 19 and 26 are in frictionalengagement with friction plates 16, 17, and thus adapted to transmit therotary motion of rotor of motor 13 to the rotor 1 4 c of motor 14.Shifting of lever 21 to the left or right, as the case may be, permits acontinuous change of the gear ratio of transmission 16, IS, 18, 2t and17, and hence a continuous change of the angular velocity at which rotoris being driven by rotor 130. Gear 4 driving gear 3 on the shaft 2 ofplayback head 1 is fixedly mounted on stator 14a for joint rotationtherewith.

Since the relative angular velocity between the stator since playbackhead 1 is being driven by the stator 14a and capstan 8 is being drivenby the rotor 14c of synchronous motor 14, the relative velocity betweenthe surface of playback head 1 and the magnetic tape 5 will be constant.The absolute velocity of tape 5 depends upon the angular velocity ofcapstan S which, in turn, depends upon the angular velocity of rotor140. The latter velocity depends, in turn, on the gear ratio oftransmission 16, 19, 18, 20 and 17 which can be changed continuously toachieve either decelerated, or an accelerated playback, as desired.

For a more complete disclosure of the structural features of playbackhead 1 reference ought to be had to FIGS. 1a to 8, inclusive.

Referring now to FIGS. la and 2, the multiple playback head showntherein comprises a substantially cylindrical magnetic core structuremade up of a pair of substantially U-shaped stacks 11 of laminations ofa magnetic material. The cross-sectional area of inner core portion 13is relatively large, and the cross-sectional area of the outer coreportions 12' decreases progressively toward the air gaps formedtherebetween. The juxtaposed ends of outer core portions 12 areseparated by relatively narrow gaps and form the pole pieces of themagnet system. Each stack of laminations 11' is provided with atransversal bore 1 intended to receive a rivet firmly joining theindividual laminations togeth r so as to form a solid stack. The magnetcoil 15 may either be self-supporting, or wound upon an insulatingbobbin, as shown. The upper and lower surfaces of the core structure 11shown in FIGS. la and 2 are supposed to be provided with appropriatesupports permitting core structure 11' to be rotatably mounted (uponshaft 2, as shown in FIG. 1).

When assembling the multiple playback head shown in FIGS. la and 2, bothcore structures 11' are initially separated and then moved from oppositesides into magnet coil 15'.

The cylindrical surfaces of core structures 11' are illtended tosupport, or guide, the magnetic tape, as clearly shown in FIG. 1.

It will be apparent from a consideration of FIGS. 1, 1a and 2 that thetwo magnetic cores l l define a pair of magnetic flux paths each havinga pole gap arranged substantially in a cylindrical surface situatedcoaxially with respect to the driving shaft 2 of the playback head, andthat both flux paths link with substantially all turns of magnet coil15'. It is this single coil feature which makes it readily possible tomanufacture rotatable multiple playback heads having small spacingsbetween pole gaps without the need of resorting to excessiveminiaturization.

Referring now to FIGS. 3 to 5, inclusive, the structure shovm thereincomprises a coil N in the shape of a toroid intended to be arrangedcoaxially with respect to the driving shaft of the playback head (part 2shown in FIGS. 1 and 10). The multiple playback head shown in FIGS. 3and 4- comprises three individual playback heads which are angularlydisplaced 360/3 deg.=120 deg. Each of the three flux paths of theplayback head is made up of a pair of substantially U-shapedferro-magnet-ic elements 21', 22, shown per se in FIG. 5, engagingmagnet coil 20 in the fashion clearly shown in FIGS. 3 and 4. The threesubstantially 'U-shaped angularly displaced magnetic elements 21 engagethe radially inner surface 66, the upper surface 61 and the radiallyouter surface '62 of coil 20'. The three substantially U-shapedangularly displaced magnetic elements 22 engage the radially innersurface 60, the lower surface 63 and the radially outer surface 62 ofcoil 2%. Magnetic elements 21 and 22 form a plurality of pole pieces 21"and 22", respectively. These pole pieces are arranged outside coil 20 ina cylindrical su race coaxial with respect to coil 29' and the shaft(indicated by reference numeral 2 in FIGS. 1 and 10) supporting all theconstituent parts of the playback head. Each of the pole pieces 21 and22 and all of the pole gaps formed between immediately adjacent polepieces extend substantially along a generating line of the cylindricalsurface in which the pole pieces and the pole gaps are situated.Reference numeral 24 has been applied to indicate a body of anon-magnetic material, e.g. a synthetic resin, wherein the toroidal coil20 and the core structure 21, 22 are embedded. Body 24which may be acasting-has a cylindrical surface 2 around which the magnetic tapehaving a sound track thereon is supposed to be wound. The cylindricalsurface 24 formed by non-magnetic body 24 is substantially coextensivewith the cylindrical surface wherein pole pieces 21 and 22 are arranged.It will be apparent from the foregoing that the magnetic core structure21, 22' forms a plurality of magnetic flux paths, i.e. three magneticflux paths, each linking with all the turns of central coil 20'.

The magnetic playback head lis not provided with any shielding means forprotection against the action of magnetic fields other than thoseproduced by the magnetic tape in cooperative engagement with theplayback head. It was found that provision of magnetic shielding meansis neither necessary, nor feasible. The gap formed between pole pieces21" and 22" is in the order of .02 millimeter. A non-magnetic fillerought preferably to be provided in the gap formed between the polepieces of rotatable multiple playback heads embodying this inven tion,irrespective of the specific embodiment thereof which is being applied.

Referring now to FIG. 6, the magnetic core structure shown thereincomprises two channel-shaped magnetic elements 31 and 32. The webportions of magnetic elements 31' and 32 engage the upper and lower endsof the cylindrical core member 33. The latter is supposed 6 to besurrounded by a coaxial toroidal magnet coil (not shown), and supportedby a coaxial head driving shaft (not shown). The flange portions 34 ofchannel element 31' and the flange portions 35' of channel element 32'are arranged in a cylindrical surface substantially coaxial to the outersurface of the toroidal magnet coil supposed to surround core member 33.Flange portions 34, 35' are spaced to form a system of relatively narrowgaps 65, each situated between two of a system of relatively wide gaps66. The magnetic tape 5 (shown in 'FIG. 1) having a sound trackthereonis supposed to be wound around a predetermined portion of theperiphery of the cylindrical surface wherein flange portions 34' and 35'are arranged. The core member 33' and the two channels 31 and 32 form amagnetic core structure having a plurality of flux paths of magneticmaterial linking with all the turns of the toroidal magnet coil supposedto surround core member 33'. The structure shown in FIG. 6 may beembedded in a coaxial body of non-magnetic material, e.g. a syntheticresin, as described in connection with FIGS. 3 to 5, to provide anappropriate cylindrical surface around which the magnetic tape can bewound and by which it can be supported, as shown in FIG; 1. As analternative, the aforementioned body of non-magnetic material may beomitted, and a plurality of circular discs of non-magnetic sheetmaterial may be substituted for it, the discs forming an integral partof the rotatable multiple magnetic playback head.

The structure shown in FIG. 7 is substantially the same as that of FIG.6, except that the former has three pairs of poles and three pole gaps,and the latter but two pairs of poles and two pole gaps. The structureshown in FIG. 7 comprises a top plate 41' and a bottom plate 42 and acylindrical core member 67 providing a path of small reluctance betweenplates 41 and 42'. Plate 41' forms three magnetic arms having apredetermined geometrical configuration in engagement with the topsurface of core member 67 and extending radially outwardly therefrom.Plate 42" forms three magnetic arms having the same predeterminedgeometrical configuration as the arms formed by plate 41. The armsformed by plate 42 are in engagement with the bottom surface of coremember 67', and extend radially outwardly therefrom. The arms formed byplates 41 and 42 are angularly displaced to each other, and hence out ofregistry. Each of the arms formed by plates 4i and 42 is provided with apole piece 44 and 45, respectively. A toroidal magnet coil (not shown)coaxial with core member 67 is supposed to be arranged in the spacewhich is radially inwardly bounded by core member 67 and radiallyoutwardly bounded by pole pieces 44' and 45'. The pole pieces 44' and45' define a cylindrical surface coaxial with respect to core member 67,and each pole piece includes a substantially rectangular portion of saidcylindrical surface. To be more specific, the juxtaposed edge portionsof immediately adjacent pole pieces 44', 45 form pole surfaces, andbound pole gaps, all situated in a cylindrical surface coaxial withrespect to the core member 67 (and the toroidal magnet coil, and theheadsupporting shaft not shown in H6. 7). The magnetic tape cooperatingwith the rotatable magnetic playback head shown in FIG. 7 will be guidedby cylindrical tape guiding means of the same nature as described inconnection with FIGS. 3 to 6, inclusive.

The rotatable magnetic playback head shown in FIG. 8 is of the samenature as that shown in FIGS. 6 and 7, yet the playback head shown inFIG. 8 comprises a larger number of pole gaps, i.e. four such gaps.Reference numeral 2' has been applied to indicate the driving shaft ofthe playback head. Core member 69 is arranged inside of toroidal coil 68in coaxial relation to shaft 2' and coil 68. Reference numeral 51 hasbeen applied to indicate a plurality of channels of magnetic materialarranged in substantially the same fashion as the channels in thestructure of FIG. 6. The flange portions 200 of the channel membersshown in FIG. 8 enclose an angle other than 90 degrees with thelongitudinal planes of symmetry of the web portions, as a result ofwhich each immediately adjacent pair of flange portions 2% encloses anobtuse angle. By virtue of this specific geometry the edges of the webportions remote from the edges forming the pole gaps 53 are renderedmagnetically ineffective. Reference numeral 71 has been applied toindicate a cylindrical surface formed by a body of nonmagnetic material,such as an appropriate synthetic resin. Surface 71 is intended to engageand guide the magnetic tape 5' intended to be played back.

Assuming a magnetic tape to be moved at a velocity of 9.5 centimetersper second. The requirement that the portions of the sound track to berepeated, or omitted, as the case may be, not exceed 40 millisecondsmakes it necessary to provide a spacing of 3.8 millimeters between thepole gaps of the rotatable multiple playback head. If the playback headhas but two pole gaps, as shown in FIGS. la and 6, the diameter of theplayback head is barely 2.5 millimeters. Since it is not indicated tohave playback heads of extremely small size it is desirable to increasethe number of pairs of poles and pole gaps, as shown in FIGS. 7 and 8.

It will be apparent that the projections 34, 35' of the structure ofFIG. 6, the projections 44, 45' of the structure of FIG. 7, and theprojections 200, 200 of the structure of FIG. 8 are substantiallyequivalents. These projections and the pole-gap-forrning edges thereofhave a length which is equal to the spacing between the top and bottomplates, i.e. the spacing between parts 31, 32' in FIG. 6, the spacingbetween parts 41, '42. in FIG. 7 and the spacing between the webportions of channel elements 51' of FIG. 8. The playback heads shown inFIGS. 6 to 8, inclusive, have a minimum height for a given width ofrecording tape or, in other words, are very compact. The magnetic gapsof the structures of FIGS. 6 to 8 have a different circular pitch, thatof gaps 65 of FIG. 6 being 180 degrees, that of gaps 46 of FIG. 7 being120 degrees and that of gaps 53 of FIG. 8 being 90 degrees. In allembodiments of the invention shown the width of pole-piece-formingprojections is but a fraction of the circular pitch between pole gaps.The edges formed between the end plates of the magnetic core structureand their bent pole-piece-forrning projections are the points of maximumspacing between the playbackhead-supporting shaft and any constituentportion of said end plates, or top and bottom plates. All other edgeslaterally bounding said top and bottom plates have a smaller spacingfrom said shaft than the edges formed between the top and bottom platesand the pole-pieceforming projection thereof. Referring, for instance,to FIG. 6, the spacing of the edges formed between parts 31 and 34, orbetween parts 32 and 35 from the center of core 33' or from the centerof head-supporting shaft, respectively, is equal to half of the lengthof parts 31, or 32. The distance of the lateral edges of parts 31 and32' from the above centers is much less than half of the length of parts31 and 32'. This geometry is important to minimize leakage flux in theplayback head. The structures of FIGS. 7 and 8 are designed in a fashionsimilar to that of FIG. 6 to minimize leakage flux in the respectiveplayback head. Referring to FIG. 7, it will be apparent that the edgesformed between parts 41', 44' and parts 42, 45 have the maximum spacingfrom the center of the device. All other edges laterally bounding plates41, 42 are substantially convex and thus have a smaller spacing from thecenter of the device than the edges formed between parts 41, 4-4 andparts 42', 45. It will be also apparent from FIGS. 6 to 8, inclusive,that no portions of the top and bottom plates other than those formingthe pole pieces 34', 35; 44', 45 and 280, are bent 90 degrees out of thegeneral planes of the top and bottom plates. This is important in orderto minimize the height of, and the leakage flux Within, the playbackhead,

It will also be apparent from FIG. 8 that the projections 2% have curvedsurfaces which are not coaxial with the toroidal coil 68. The curvedcylindrical surfaces of projections 206 have axes of curvature arrangedeccentrically with respect to the axis of coil 68, and with respect tothe playback-head-supporting shaft 2'.

Referring now to FIG. 9, numeral 1 has been applied to indicate arotatable multiple playback head such as illustrated in FIGS. 1a to 8,inclusive. Head 1 is supported by head shaft 2 mounted on frame plate12. Head shaft 2 is operated by gears 3, 4 of which the latter forms anintegral part of the rotatable stator 14a of motor 1 t. Motor 14comprises in addition to the part 14a referred to as the stator, a part140 referred to as the rotor of motor 14. Motor 14 may be a synchronousmotor, or an asynchronous motor. It must, however, be a motor of thetype wherein the relative angular velocity between rotor 14c and stator14a is fixed, or constant. As mentioned above, stator 14a operatesplayback head 1 by the intermediary of gears 3, 4. Rotor 140 is mountedon shaft 9 for operating a tape-advancing capstan (see FIG. 1). Motor 13is substantially identical to motor 14, except that the former comprisesbut one part rotatable relative to frame or chassis 12, 15, i.e. therotor 13c. Rotor 13c and rotor 140 are each fixedly coupled with afriction disk 16 and 17, respectively. Shaft supporting roller 81 isarranged at right angles to shafts 13b and 9, and adapted to be shiftedselectively either to the left, or to the right, as indicated by arrowsin FIG. 9. The rotation of the rotor 130 of motor 13 is transmitted bymeans of friction gears 16, 8d and 17 to the rotor 14c of motor 14.Friction gears 16, 81 and 17 permit a continuous variation of gearratio, and hence of the speed, superimposed by rotor 13a of auxiliarymotor 13 upon the rotor 14c of main motor 14.

The structure of FIG. 10 is substantially identical with that of FIG. 9and the same reference characters have been applied in both figures toindicate like parts. FIG. 10 differs from FIG. 9 only inasmuch as thetransmission between parts 16 and 17 is concerned. FIG. 10 shows a shaftH8 at right angles to shafts 12b and 1311 supporting friction rollers 19and 20, of which one engages friction disc 16 and the other frictiondisc 17. Shaft 18 is supported by lever 21 movable selectively to theleft or right, as indicated by arrows 22, 23 whereby the gear ratio maycontinuously be varied at will.

The structure shown in FIG. 11 is substantially identical to thestructure shown in FIGS. 9 and 10 and, therefore, needs to be describedonly to the extent to which it differs from the structures illustratedin the last referred to figures. The motor 96 shown in FIG. 11 comprisesa stator 943a and a rotor 900. Gear 91 forms an integral part of statora and drives gear 92 on the shaft of which the rotatable multiplemagnetic playback head 93 is mounted. The rotor 900 of motor 9% has adriving shaft 96 by which a tape advancing capstan (not shown) issupposed to be driven. The shaft 95 of an auxiliary motor generallyindicated by numeral 94 is arranged parallel to the shaft 96 of mainmotor 90. Reference numeral 97 has been applied to indicate a surface inthe shape of an inverted truncated cone forming an integral part of therotor of motor 94. The rotor 900 of motor 99 is in the shape of anidentical truncated cone, i.e. an identical truncated cone surface formsan integral part of the rotor 900 of motor 90. Truncated cone surfaces97 and 90 are arranged in such a way as to have a pair of parallelgenerating lines 99* and 10f), respectively, at the juxtaposed sidesthereof. Shaft lfl ll is mounted parallel to generating lines 99' and1M, and equally spaced from each of these lines. Friction wheel 162 isin frictional engagement with both truncated cones 97 and 980, andtransmits power from the rotor of motor 94 to the rotor of motor 9G.Parts 97, 192 and @llc form a continuous variable speed drive enablingto control at will the velocity of capstan operating shaft 96 byshiftingfriction gear 102 either upwardly, or downwardly, as indicatedby the two arrows in FIG. 11.

Referring now back to FIG. 1, it will be apparent that there is apredetermined portion of the cylindrical periphery of playback head 1which is being engaged by magnetic tape 5. Magnetic tape 5 must never bein engagement with more than two pairs of adjacent poles, i.e. with notmore than two pole gaps. The angle along which the tape engages thecylindrical surface of the rotatable multiple playback head must not besubstantially less than the angle enclosed between adjacent pole gaps.It has been found that if the magnetic playback head has four pairs ofpoles, and consequently four pole gaps, the angle along which the tapeengages the cylindrical surface of the rotatable multiple playback headshould preferably be 86 degrees. This has been indicated in FIG. 8 wherereference numeral 5' has been applied to indicate the magnetic tapehaving a sound track thereon and reference numeral '71 has been appliedto indicate the cylindrical tape-guiding surface of the multipleplayback head.

The motors used to drive the magnetic playback head and the tapeadvancing capstan are provided with slip rings to connect the motors toa suitable source of power. FIGS. 1, 9 and 10 indicate slip rings 2G1forming an in tegral part of the main motor. In a similar way therotatable multiple magnetic playback head proper must be supplied withslip rings which have not been shown in the drawings. The auxiliarymotors 13 of FIGS. 9 and 10 do not require slip rings. The same is truein regard to motor 94 of FIG. 11. Motor W of FIG. 11

7 requires slip rings not shown in this figure.

While in accordance with the patent statutes, I have disclosed thedetails of several preferred embodiments of my invention, it is to beunderstood that many of these details are merely illustrative andvariations in their precise form will be possible or necessary dependingupon the particular nature of application. I desire, therefore, that myinvention be limited only to the extent set forth in the appended claimsand by the prior art.

I claim as my invention:

1. A sound reproducing system adapted to control playback speedcomprising a driving shaft, a toroidal magnet coil arrangedsubstantially coaxially with respect to said shaft, a magnetic corestructure supported by said shaft supporting said coil, said corestructure including a core member arranged inside said coilsubstantially coaxially with respect to said shaft, a plurality ofsubstantially channel-shaped magnetic elements, web portions of saidplurality of elements engaging the upper and lower ends of said coremember and flange portions of said plurality of elements being arrangedradially outwardly from said core member and spaced to form a system ofrelatively narrow gaps and to form a system of relatively wide gaps eachsituated between two of said system of relatively narrow gaps, saidflange forming a plurality of spaced magnet poles situated in acylindrical surface substantially coaxial to said coil and extendingparallel to said shaft, and a magnetic tape having a sound track thereonpositioned in a surface parallel to said shaft and wound in a directionlongitudinally thereof around a predetermined portion of the peripheryof said cylindrical surface wherein said magnet poles are arranged.

2. In a sound reproducing system adapted to control playback speed thecombination of a driving shaft; a toroidal magnet coil arrangedsubstantially coaxially with respect to said shaft; a magnetic corestructure supported by said shaft supporting said coil, said corestructure including a core member arranged inside said coilsubstantially coaxially with respect to said shaft, said core structurefurther including a first system of angularly displaced channel elementsjuxtaposed with the web portions'thereof to one of the end surfaces ofsaid coil and overlapping with the flange portions thereof the lateralsurface of said coil, said core structure further including a secondsystem of angularly displaced channel elements juxtaposed with the webportions thereof to the other of the end surfaces of said coil andoverlapping with the flange portions thereof said lateral surfaces ofsaid coil, said first system of channel elements and said second systemof channel elements being angularly displaced to each other and saidflange portions of said first system and said flange portion of saidsecond system defining a plurality of narrow magnetic gaps; and amagnetic tape having a sound-track thereon position in a surfaceparallel to said shaft and wound in a longitudinal direction around aportion of said lateral surface of said coil.

3. A multiple recording and playback head for sound recording andreproducing systems comprising a plurality of parts arranged within andbounding a cylindrical space defined by an upper end plane, a lower endplane parallel to said upper end plane and a lateral cylindricalboundary surface forming an upper circular edge at the locus ofintersection thereof with said upper end plane and forming a lowercircular edge at the locus of intersection thereof with said lower endplane, said plurality of parts including a shaft coextensive with theaxis of said space; a toroidal magnet coil arranged substantiallycoaxially with respect to said shaft; and a magnetic core structuresupported by said shaft supporting said coil, said core structureincluding a core member arranged inside said coil substantiallycoaxially with respect to said shaft; said core structure furtherincluding a first plurality of arms extending radially outwardly fromthe region of said shaft, having a predetermined circular pitch, beingarranged in said upper end plane and having lateral edges generallyspaced from said upper circular edge; a second plurality of armsextending radially outwardly from the region of said shaft, having apredetermined circular pitch, being arranged in said lower end plane andhaving lateral edges generally spaced from said lower circular edge;said first plurality of arms and said second plurality of arms beingcongruent and angularly displaced relative to each other; the radiallyouter end of each of said first plurality of arms being approximatelycoextensive with a portion of said upper circular edge and the radiallyouter end of each of said second plurality of arms being approximatelycoextensive with a portion of said lower circular edge; the radiallyouter end of each arm of said first plurality of arms and the radiallyouter end of each arm of said second plurality of arms being providedwith an integral pole projection arranged approximately in said lateralboundary surface; the pole projections of said first plurality of armsand the pole projections of said second plurality of arms having edgessituated approximately in said lateral boundary surface and adapted todefine a cyclic pattern of narrow gaps and of gaps having a width in theorder of said circualr pitch of said first plurality of arms and of saidsecond plurality of arms.

4. A multiple recording and playback head as specified in claim 3,wherein said pole projection of each of said first plurality of arms andof each of said second plurality of arms has a curved surface having anaxis of curvature arranged eccentrically with respect to said shaft andslightly deviating from said lateral boundary surface.

5. A multiple recording and playback head for sound recording andreproducing systems comprising a plurality of parts arranged within andbounding a cylindrical space defined by an upper end plane, a lower endplane parallel to said upper end plane and a lateral cylindricalboundary surface forming an upper circular edge at the locus ofintersection thereof with said upper plane and forming a lower circularedge at the locus of intersection thereof with said lower end plane,said plurality of parts including a shaft coextensive with the axis ofsaid space; a toroidal magnet coil arranged substantially coaxially withrespect to said shaft and jointly rotatable with said shaft; and amagnetic core structure supported by and jointly rotatable with saidshaft; said core structure including a core member araomers rangedinside said coil; said core structure further including a firstplurality of arms extending radially outwardly from the region of saidshaft, enclosing equal predetermined angles, being arranged in saidupper end plane and having lateral edges generally spaced from saidupper circular edge; a second plurality of angularly displaced armsextending radially outwardly from the region of said shaft, enclosingequal predetermined angles, being arranged in said lower end plane andhaving lateral edges generally spaced from said lower circular edge;said first plurality of arms and said second plurality of arms beingcongruent and being angularly displaced from each other; the radiallyouter end of each arm of said first plurality of arms being providedwith an integral pole projecting bent about 90 degrees out of said upperend plane and the radially outer end of each arm of said secondplurality of arms being provided with an integral pole projection bentabout 90 degrees out of said lower end plane; each pole projection ofsaid first plurality of arms and each pole projection of said secondplurality of arms having pole-surface-forming edges equal in length tothe entire spacing between said upper end plane and said lower end planeand defining a cyclic system of pole gaps.

6. A multiple sound recording and sound playback head comprising aplurality of parts arranged within and bounding a cylindrical spacedefined by an upper end plane, by a lower end plane parallel to andhaving a predetermined spacing from said upper end plane and by alateral cylindrical boundary surface forming an upper circular edge atthe locus of intersection thereof with said upper end plane and forminga lower circular edge at the locus of intersection thereof with saidlower end plane, said plurality of parts including a shaft coextensivewith the axis of said'space; a toroidal magnet coil arrangedsubstantially coaxially with respect to, and jointly rotatable with,said shaft; and a magnetic core structure supported by and jointlyrotatable with said shaft, said core structure including a core memberarranged inside said coil, said core structure further including a firstplurality of arms extending radially outwardly from the region of saidshaft, having a predetermined circular pitch, being arranged in saidupper end plane and having lateral edges generally spaced from saidupper circular edge; a second plurality of arms extending radiallyoutwardly from the region of said shaft, having a predetermined circularpitch equal to said first mentioned circular pitch, being arranged insaid lower end plane and having lateral edges generally spaced from saidlower circular edge; said first plurality of arms and said secondplurality of arms having a predetermined angular displacement, and theradially outer ends of the constituent arms of said first plurality ofarms and the radially outer ends of the constituent arms of said secondplurality of arms being provided with integral pole projections beingarranged substantially in said lateral boundary surface; said poleprojections having edges adapted to define a cyclic pattern of narrowand wide pole gaps substantially equal in length to said spacingbet-ween said upper plane and said lower plane, and the width of saidwide pole gaps being approximately equal to said circular pitch.

7. A multiple sound recording and playback head comprising a pluralityof parts arranged within and bounding a cylindrical space defined by anupper end plane and a parallel lower end plane spaced from said upperend plane and by a cylindrical boundary surface intersecting with saidupper end plane along an upper circular edge and intersecting with saidlower end plane along a lower circular edge, said plurality of partsincluding a driving shaft coextensive with the axis of said space; atoroidal magnet coil arranged coaxially with respect to said shaft andjointly rotatable with said shaft; a magnetic core structure supportedby and jointly rota-tatble with said shaft; said core structureincluding means defining a flux path inside said coil; said corestructure further including a top plate arranged in said up er end planeand having edge portions 12 receding from said upper circular edge todefine a plurality of upper arms extending radially outwardly from theregion of said shaft and having a predetermined circular pitch; saidcore structure including a bottom plate arranged in said lower end planeand having edge portions receding from said lower circular edge todefine a plurality of lower arms extending radially outwardly from theregion of said shaft and having the same circular pitch as saidplurality of upper arms; the radially outer ends of said plurality ofupper arms being substantially coextensive with portions of said uppercircular edge and the radially outer ends of said plurality of lowerarms being substantially coextensive with said lower circular edge; eachconstituent arm of said plurality of upper arms l and each constituentarm of said plurality of lower arms eing provided with an inte ral poleprojection enclosing an angle of degrees with said upper end surface andwith said lower end surface; the pole projections of said plurality ofupper arms and the pole projections of said plurality of lower armshaving edges parallel to said shaft situated approximately in saidlateral boundary surface and being adapted to define a cyclic pattern ofrelatively narrow axial pole gaps and of relatively wide axial polegaps, and said wide axial pole gaps being approximately equal in widthto the extent of said circular pitch.

8. A sound reproducing system adapted to control playback speedcomprising a driving shaft, a toroidal magnet coil arrangedsubstantially coaxially with respect to said shaft, a magnetic corestructure supported by said shaft supporting said coil, said corestructure including a first plurality of angularly spaced magnetic armssituated above said coil extending from points adjacent said shaftsubstantially radially outwardly to points remote from said shaft, :1second plurality of angularly spaced magnetic arms situated below saidcoil extending from said points adjacent said shaft substantiallyradially outwardly to points remote from said shaft and angularlydisplaced with respect to said first plurality of arms, magnetic pathmeans situated inside said coil providing a magnetic connection betweensaid first plurality of arms and said second plurality of arms, each ofsaid first plurality of arms and each of said second plurality of armscomprising a pole piece arranged outside said coil substantially in acylindrical surface coaxial with respect to said shaft and each saidpole piece having a leading and a trailing polesurface-forming edgeparallel to said shaft and equal in length to the axial spacing of saidfirst plurality of arms from said second plurality of arms, and amagnetic tape having a sound track thereon positioned in a surfaceparallel to said shaft and wound in a direction longitudinally thereofto engage said cylindrical surface along a predetermined angle less than360 degrees.

9. A sound reproducing system as specified in claim 8 comprising a bodyof non-magnetic material wherein said coil and said core structure areembedded, said body having a cylindrical surface around which said tapeis wound, enveloping said leading and said trailing edge of each saidpole piece.

10. A sound reproducing system adapted to control playback speedcomprising a driving shaft, a toroidal magnet coil arrangedsubstantially coaxially with respect to said shaft, a magnetic corestructure supported by said shaft supporting said coil, said corestructure including a core member arranged inside said coilsubstantially coaxially with respect to said shaft, 2. first pluralityof angularly spaced magnetic arms having a predetermined geometricalconfiguration in engagement with the top surface of said core memberextending radially outwardly therefrom, a second plurality of angularlyspaced ma netic arms having the same predetermined geometricalconfiguration as said first plurality of arms in engagement with thebottom surface of said core member and extending radially outwardlytherefrom, said second plurality of arms being angularly displaced inregard to and out of registry with said first plurality of arms, each ofsaid first plurality of arms and each of said second plurality of armsbeing provided with a pole piece situated outside said coilsubstantially in a cylindrical surface coaxial to the outer surface ofsaid coil and each having a leading and a trailing pole-surface-formingedge parallel to said shaft and equal in length to the axial spacing ofsaid first plurality of arms from said second plurality of arms, and amagnetic tape having a sound track thereon positioned in a surfaceparallel to said shaft and wound in a direction longitudinally thereofaround a predetermined portion of the periphery of said cylindricalsurface.

References Cited in the file of this patent UNITED STATES PATENTS UNITEDSTATES PATENT OFFICE CERTIFICATE OF CORRECTIQN Patent No. 3,047,673 July31, 1962 Anton M. Springer It is hereby certified that error appears inthe above numbered patant requiring correction and that the said LettersPatent should read as corrected below Column 4, line 42, after "stator"insert and the rotor of a synchronous motor is constant, and column 10,line 11 for "position" read positioned line 54, for "circualr" readcircular column 11, line 14, for "projecting" read projection Signed andsealed this 24th day of December 1963.

(SEAL) Att st: EDWIN L. REYNOLDS ERNEST W. SWIDER Attesting Officer Acting Commissioner of Patents

